Two general categories of rigs include drilling rigs and service/workover rigs. Drilling rigs are used to drill wells (e.g., oil, natural gas), while service/workover rigs are used to service or work existing wells for any appropriate reason. Representative servicing or workovers of existing wells includes without limitation replacing one or more components (including downhole components) associated with the well (e.g., tubing, valves, seals, flanges, blowout preventers), directing one or more components into the well for any appropriate purpose (e.g., a tool for opening a downhole blockage), executing one or more well operations (e.g., fracturing, acidizing), or the like.
Both drilling and service/workover rigs typically use a derrick that supports one or more pulleys, one or more block and tackles, or the like. Various lines, cable, or the like may be directed around one or more of these pulleys/block and tackles to lift the desired component(s) and/or to lower the desired component(s) as desired/required. These lines or cables are anchored to what is commonly referred to in the art as a drawworks. An appropriate power source (e.g., a right angle drive) rotates one or more drums of the drawworks in one direction to wind the line/cable around one or more drums of the drawworks to lift the desired component(s), while the power source rotates one or more drums of the drawworks in the opposite direction to unwind the line/cable from one or more drums of the drawworks to lower the desired component(s). “Cable” is commonly viewed as being of a heavier grade than “line,” and thereby more appropriate for handling heavier components. Cable is commonly associated with a main drum of a drawworks, while line is commonly associated with a sand drum of a drawworks. Service or workover rigs use a drawworks having both a main drum and a sand drum, while drilling rigs typically only use a main drum.
There are various deficiencies with at least certain existing drawworks designs. One is lubrication. Regular lubrication is of course required to maintain various components of the drawworks in proper working order, including various bearings. One known drawworks uses a plurality of individual slots that are spaced about the perimeter of a shaft on which the noted cable/line is wound, and further that extend along at least a portion of the length of this shaft. Lubricant that is directed into an individual slot travels along the slot and to one or more bearings. The volume of each individual slot is relatively small in that the depth of each individual slot on the exterior of the shaft is relatively shallow. Therefore, the individual slots are prone to becoming block, for instance due to “waxing” or solidification of lubricant within the slot.
At least certain drawworks designs are somewhat problematic in relation to maintenance. The main drum and the sand drum are of a different size in the case of all known drawworks. Therefore, many of their parts are not interchangeable, such as the drive sprockets that rotate the shaft to wind or unwind the cable or line. Moreover, one or more components of at least some drawworks are permanently joined together to define an assembly. This is potentially deficient in a number of respects. One is that the entire assembly may need to be removed to access/replace one or more components. This entire assembly also may be quite heavy and difficult to handle. Another is that if one of the components of the assembly breaks or becomes unsuitably worn, the entire assembly may very well need to be replaced. In this regard, it is common for at least some drawworks to use a pair of drive sprockets that are welded together to define a drive sprocket assembly, where each individual drive sprocket has a set of gear teeth about its perimeter, and further to weld this drive sprocket assembly to a common drive hub. The sand drum of at least some drawworks have a drive sprocket assembly as described welded to each side of a common drive hub, such that the pair of drive sprocket assemblies sandwiches a flange of the drive hub between the pair of drive sprocket assemblies.